The invention relates to a process chain for machining an initially cylindrical bore by honing tools as well as to the corresponding honing tools.
The manufacturers of motor vehicles are faced with the permanent task of continuously reducing the fuel consumption of their vehicle fleet furnished with reciprocating piston engines. In reciprocating piston engines, the friction between the piston or the piston rings, on the one hand, and the cylinder bore plays a great part in the internal friction losses up to about 35%. Therefore, the reduction of friction in the area of the cylinder bore provides a significant potential for reducing the fuel consumption.
An approach for reducing the friction between piston and cylinder bore is form honing, developed by the applicant, that is disclosed in detail in EP 2 170 556 B1. In this method, the deviations from the geometry of a cylinder that are caused by clamping actions during assembly and/or thermal expansions of the cylinder bore are equalized in that complementary elevations or depressions are formed during form honing. This method is very effective and is used successfully in the production of various reciprocating piston engines.
DE 10 2013 204 714 A1 discloses a honing method by means of which the cylinder bore of an internal combustion engine is provided with a bottle shape. A shape in which the cylinder bore has two cylindrical sections that have a different diameter is referred to as bottle shape. The section with the smaller diameter is provided in the area of the cylinder head while the section with the greater diameter is provided in the area of the crankshaft. Between these areas, a truncated cone-shaped transition area is formed which occupies approximately 5% to 20% of the bore length.
DE 103 581 50 A1 discloses a method with which two sections of different hardness that are sequentially arranged in axial direction of a cylinder bore can be machined. This method and the corresponding tool are in particular advantageously employed when the cylinder bore is hardened in sections thereof and, as a consequence, the hardened section and section that is not hardened must be honed in a different way.
From this publication it is known to design the cylinder bore at its open end, i.e., where later on the cylinder head is mounted, such that a counterbored bore results. This is graphically illustrated in FIG. 6 of this publication. A slight widening in the upper section of the cylinder bore is referred to as a counterbore. In this context, this widening only applies to the uppermost quarter of the cylinder bore.
A further approach for reducing the friction between piston rings and cylinder running surface provides widening of the cylinder bore at the end that is located in the vicinity of the crankcase. This end is also referred to in the following as “bottom end”. The later published DE 10 2015 109 609 discloses a process chain for widening (cylinder) bores at their bottom end. For this purpose, relatively short honing stones are employed.
In order to reduce the pump losses of internal combustion engines, the cylinders of internal combustion engines of the newest generation have one or more recesses at their bottom end. These recesses reduce the pump losses. However, they lead to the honing process being carried out with an interrupted cut. The tools known from the prior art with short honing stones that are suitable for widening a bore cannot be used in bores with a recess because the short honing stones have the tendency to cant when they “travel across” the recess.
The invention has the object to provide honing methods, a process chain as well as suitable honing tools for performing the methods which allow for the inexpensive and reproducible manufacture of widened cylinder bores with at least one recess. This widening is referred to also as “conical honing”.
In this context, the desired geometry of the “cylinder bore” is a cylindrical section with an adjoining conically widened end. Possibly, the bore also comprises one or two additional cylindrical sections. A truncated cone-shaped (cylinder) bore in the meaning of the invention is a bore whose diameter changes continuously across more than half of the length, preferably at least ¾ of the length of the cylinder bore. Ideally, the truncated cone occupies more than 85%, in some cases even 100%, of the length of the cylinder bore.